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Abstract

A novel fast frequency-based method to estimate the focus distance of digital hologram for a single object is proposed. The focus distance is computed by analyzing the distribution of intersections of smoothed-rays. The smoothed-rays are determined by the directions of energy flow which are computed from local spatial frequency spectrum based on the windowed Fourier transform. So our method uses only the intrinsic frequency information of the optical field on the hologram and therefore does not require any sequential numerical reconstructions and focus detection techniques of conventional photography, both of which are the essential parts in previous methods. To show the effectiveness of our method, numerical results and analysis are presented as well.

Fig. 2 The wave profiles, (a) the amplitude and (b) the phase, for a hologram of for a planar cross-shaped object with the focus distance z0 = 1 and (c) the numerical reconstruction at the estimated focus distance z̃ = 1.00736.

Fig. 7 The wave profiles, (a) the amplitude and (b) the phase, for a phase-shifting hologram of a real coin and (c) the numerical reconstruction result at the estimated focus distance z̃ = 0.356507(brightness enhanced).